Для данила

src/main/java/ru/IP_LabWorks/IP/University/Model/Group.java

@@ -2,6 +2,9 @@ package ru.IP_LabWorks.IP.University.Model;
 
 import jakarta.persistence.*;
 
+import java.util.List;
+import java.util.Objects;
+
 @Entity
 @Table(name = "groups")
 public class Group {
@@ -9,4 +12,34 @@ public class Group {
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     private Long id;
     private String name;
+    @OneToMany
+    @JoinTable(name = "students_in_groups",
+            joinColumns = @JoinColumn(name = "group_id"),
+            inverseJoinColumns = @JoinColumn(name = "student_id"))
+    private List<Student> students;
+
+    public Long getId() { return id; }
+    public String getName() { return name; }
+    public void setName(String name) { this.name = name; }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        Group group = (Group) o;
+        return Objects.equals(id, group.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id);
+    }
+
+    @Override
+    public String toString() {
+        return "Group{" +
+                "id=" + id +
+                ", name='" + name + '\'' +
+                '}';
+    }
 }

src/main/java/ru/IP_LabWorks/IP/University/Model/Student.java

@@ -18,6 +18,9 @@ public class Student {
             inverseJoinColumns = @JoinColumn(name = "subject_id"))
     private List<Subject> subjects;
 
+    public Student() {
+    }
+
     public Student(String name, Integer age) {
         this.name = name;
         this.age = age;
@@ -36,4 +39,18 @@ public class Student {
         Student student = (Student) o;
         return Objects.equals(id, student.id);
     }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id);
+    }
+
+    @Override
+    public String toString() {
+        return "Student{" +
+                "id=" + id +
+                ", name='" + name + '\'' +
+                ", age='" + age + '\'' +
+                '}';
+    }
 }

src/main/java/ru/IP_LabWorks/IP/University/Model/Subject.java

@@ -15,6 +15,9 @@ public class Subject {
     @ManyToMany(mappedBy = "subjects")
     private List<Student> students;
 
+    public Subject() {
+    }
+
     public Subject(String name) {
         this.name = name;
     }
@@ -30,4 +33,17 @@ public class Subject {
         Subject subject = (Subject) o;
         return Objects.equals(id, subject.id);
     }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id);
+    }
+
+    @Override
+    public String toString() {
+        return "Subject{" +
+                "id=" + id +
+                ", name='" + name + '\'' +
+                '}';
+    }
 }